A Slight Error In Geography Movie title notwithstanding, if there had been a meltdown at Three Mile Island, it would properly have been called a case of "Diamintina Syndrome."

If the Pennsylvania nuclear plant had melted straight through the earth, it would not have emerged anywhere near China but rather in the Diamantina Trough, a section of the Indian Ocean about 1,000 miles off the southwest tip of Australia.

Despite the popular notion about "digging straight through to China," the antipodal point on the globe for any spot in North America is somewhere in the Indian Ocean.

Of all the words in the lexicon of nuclear power, none is more hideous than "meltdown," an event so sinister that many physicists find it distasteful to talk about.

It is as simple as it is sinister: the nuclear fuel overheats to such a degree that it melts through the floor of the reactor vessel and the rock beneath it, resulting in the release of deadly radioactive contamination into the air or water.

By the night of Friday, March 30, the third day of the Three Mile Island incident, the possibility of a meltdown had become frighteningly real.

But that wasn't all. There was also a chance that the reactor might explode with enough force to shatter the containment and shower nuclear rain over a Pennsylvania countryside beginning to show signs of spring.

All of this because of a bubble.

This bubble had appeared late Thursday or early Friday. At first the Metropolitan Edison people believed it was a steam bubble. Then the experts from Babcock & Wilcox, which built the plant, and the Nuclear Regulatory Commission agreed that, with pressures of up to 1,000 pounds per square inch in the reactor, it couldn't be a steam bubble. A steam bubble would have collapsed.

That left only one possibility: a gas bubble containing hydrogen, temperamental, volatile hydrogen.

The bubble, 1,000 cubic feet and growing, would make Saturday the worst day of the crisis.

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Shortly after 1 p.m. on Friday, a giant Sikorsky helicopter with Air Force markings began circling Three Mile Island, then put down in a cornfield behind the Met Ed command post on the west bank of the Susquehanna River. A tall husky man with thinning, sandy hair and long sideburns jumped out, followed by nearly a dozen aides.

Earlier, when the chairman of the NRC, Joseph Hendrie, briefed President Carter on the situation, the president had asked for one good man at the scene, someone who could speak authoritatively for the government about what was going on. He's on his way, Mr. President, Hendrie replied. His name is Harold Denton.

Terrible events have a way of distorting and enlarging personalities, of turning unknowns into heroes. At Three Mile Island there would be no heroes, but in the tension and confusion of the weekend, Harold Ray Denton, a Washington bureaucrat, would come to symbolize a kind of technocratic reasonableness that helped to ease the public mind.

As he arrived at the command post, Denton also knew he was there to end as much as possible the public bickering between the company and the government. "We plan to work very closely with the governor," Denton said as he walked past the guards at the door of the command post.

He did not mention Metropolitan Edison.

The specialists he had brought included some of the nation's top experts in reactor safety, in meteorology (to track wind patterns in hopes of keeping an explosive shower from drifting too far), and  more ominously  in treating victims of nuclear exposure. More specialists were on the way from Washington.

Denton's team was ushered into a windowless room in the command post for a briefing from Met Ed's John Herbein. He assured Denton things were under control. At about that time, monitors in the NRC's mobile trailer measured a 90-millirem "spike" of radiation. Two workers in the plant had uncoupled a hose full of radioactive water. They were quickly hustled off the site to be checked for radiation.

Herbein's briefing continued, but just as he raised the question of coexistence between the NRC and Met Ed, Denton was summoned to a greenhouse next door.

It was the president calling.

Denton found himself momentarily at a loss for words. "I relayed to him whatever I knew at the time," he recalled. "He wanted me to get on top of the situation, to keep him informed, and provide the full resources of the government to do whatever was necessary to protect the public health and safety."

Back in Washington, the bureaucracy under presidential assistant Jack Watson had begun to do the same. Because of the danger of possible radiation contamination, Watson's office authorized the Food and Drug Administration to contract for the manufacture, packaging and shipping to Harrisburg of 240,000 one-ounce vials of potassium iodide. It could be administered orally to collect in the thyroid, hopefully saturating the gland with this non-radioactive and non-cancer-causing agent before any radioactive iodine could reach it.

For evacuation planning, Watson had dispatched Robert Adamcek, a regional director of the Federal Disaster Assistance Administration, from Philadelphia to Harrisburg to coordinate with Gov. Richard Thornburgh. He had sent John McConnell, assistant director of the Defense Civil Preparedness Agency, to consult with county civil defense officials in the region surrounding the plant.

Watson's office also coordinated the shipment of 70 tons of lead bricks from depots around the country to the Three Mile Island plant. Dosimeters, blankets and cots were gathered and sent to centers that could be used in the event of evacuation.

These preparations, according to quick estimates, cost the government $1.7 million.

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Denton hung up after conferring with the president, then walked out to the lawn in front of the house to brief the press. Before he could begin, a television reporter asked him to move  once, twice, a third time  so that the huge cooling towers of the nuclear plant formed the backdrop for the cameras.

Denton's remarks to the press were short. The White House, he said, was concerned that the reports being given the public weren't "hard, firm facts." To dispel this concern, Denton said he would send his team of experts onto the island for a first-hand inspection. They would report to the governor early in the evening, and a press conference would follow around 8:30 p.m.

But there was much to learn on the island and they stayed longer than expected. It was nearly 8 p.m. before the Denton team arrived at Thornburgh's off ice to give the governor his first full report.

He would hear two pieces of alarming news.

The first was that the reactor core was more badly damaged than first believed  at least one-third of it. This had occurred on Wednesday when the falling coolant levels in the core exposed the top of the fuel rods. Unprotected by cooling water, the cladding on the outside of the fuel rods heated up rapidly. The zirconium in the cladding oxidized, releasing more heat, which in turn ballooned and split the cladding, allowing radioactive gases like xenon-133, krypton-85 and iodine131 to seep out through the cracks.

NRC investigators had first learned of the extent of the damage to the core when they got back an analysis of the primary coolant sample. The damage, according to Denton, suggested that the radioactivity in the system of water in the containment was "hotter than hell."

The second alarming development was the gas bubble containing hydrogen, 1,000 cubic feet in size, at the top of the reactor. The reactor had become so hot that the coolant water had decomposed into its primary elements: oxygen and hydrogen.

The biggest danger was the possibility that the bubble would continue to grow, forcing all the coolant water out of the reactor, allowing the temperature of the fuel rods to build up until they reached 5,000 degrees.

At that heat, the uranium would begin to melt.

Short of the meltdown, there was the possibility of an explosion, either in the containment building or in the reactor core. On the first day of the accident, there had been a small hydrogen explosion in the containment  an event Met Ed officials didn't tell state or federal authorities about. When NRC experts found out, they launched an immediate effort to analyze the physical chemistry of the bubble.

Thornburgh was told that the NRC's analysis showed that the hydrogen could become flammable or explosive in a matter of days.

A Princeton University scientist calculated that the energy in the bubble was enough to set off an explosion equal to three tons of TNT. Such a force could rip the top of the reactor dome right off, flooding the containment with radioactive debris. There were also fears that the hydrogen would escape to the containment and explode there. One engineer calculated that a hydrogen explosion three times the force of Wednesday's blast might break the four-foot- thick walls of the containment, releasing radioactive material into the air.

Thornburgh wanted to know about the worst case, a meltdown.

Once the core reaches 5,000 degrees, he was told, the rods begin to melt, and once the melting is under way, the heavy metals like uranium and strontium begin to run right through the floor of the reactor.

Even after a loss of coolant, there would still be a four-foot pool of water below the fuel rods at the bottom of the reactor vessel that had not boiled off. The melting core would fall into that pool of water, possibly producing a steam explosion that would blow the reactor dome off like a missile and break through the containment walls.

Meanwhile, the molten core would continue to bore down through the concrete floor of the containment and into the ground. A report on reactor safety prepared by Dr. Norman Rasmussen of MIT talks about the core boring at least 40 feet into the earth, stopping only when enough rock and soil have mixed with it to dissipate its heat.

In the worst possible case, a meltdown at Three Mile Island that forced a break in the containment and poured radioactive debris into the atmosphere could trigger a catastrophe.

One of the first and most abundant fission products released in a meltdown would be a swarm of radioactive iodine, a mix of gas and liquid that could be carried far downwind in the plume escaping the plant. People living downwind would almost surely inhale some of the iodine. Rain would bring more of the iodine out as a kind of fallout, settling on the ground and dosing anybody nearby with as much as 150 rems of radiation in a single day.

The lethal dose is described as 400 rems, but the sick, the elderly, young and unborn children could easily die from a dose of 150 rems. A dose that strong could begin to kill bone marrow so fast that death might follow in a matter of months.

To many scientists, the worst consequence of an overdose of radioiodine is not the lethal dose a few might get. It is the non-lethal dose which would concentrate in the thyroid gland in the throat, where radiation might produce tumors in thousands of people over a period of 30 years.

MIT's Rasmussen postulated that in the worst possible case as many as 60,000 thyroid operations would be required over 30 years to cure the aftereffects of a massive radioiodine fallout.

The long-term effects of a meltdown are felt if radioactive cesium and strontium get into the air and water. They contaminate the land for years to come.

The half-life of cesium-137 is almost 30 years, during which time it emits gamma rays so penetrating that no living thing could survive for long on ground badly contaminated by cesium fallout.

Denton's briefing of Thornburgh ran 90 minutes. Despite the seriousness of the situation at the plant, there was no imminent danger that would force an evacuation. Thornburgh was relieved.

At 10 p.m. he and Denton arrived in the cramped press room on the sixth floor of the Capitol. Public television carried the briefing live to hundreds of thousands of homes.

Thornburgh spoke briefly, to say he would order no evacuation at that time but would reconsider as events warranted. Then he turned it over to Denton.

"This is easily the most serious situation in the life of the reactor program," Denton said. And in the next few days, he said, the federal government, not Metropolitan Edison, would be making the crucial decisions.